German Utility Model Publication No. DE 20 2009 014 490 U1 discloses a drive system for a non-railbound vehicle including an electric drive. The electric drive is a combination of at least one non-permanent magnet electric motor and at least one permanent magnet synchronous motor. A mechanical decoupling unit is arranged downstream of the latter at the drive end. In the event of a fault, said mechanical decoupling unit allows the synchronous motor to be disconnected from the unit to be driven. The decoupling unit is designed, in particular, as a one-way clutch which transmits torque to the driven unit in one direction only.
When electric motors are used for driving a machine, a short circuit in the electric motor can result in braking forces acting on the rotor of the electric motor. The shaft initially driven by the motor transmits such braking forces to the drive end, causing an abrupt change in the behavior of the machine, as a result of which problems can occur. If, in particular, the machine is a wheeled vehicle, and the electric motor drives at least one wheel of the wheeled vehicle, then a short circuit in the electric motor ultimately results in a braking torque acting on the wheel. This impairs the driving stability of the wheeled vehicle and poses a significant problem in terms of road safety. This problem is particularly serious in the case of vehicles in which an electric motor having permanent magnets is used, because a permanent magnet cannot be turned off; which means that, according to Lenz's law, its magnetic field, together with the conductor windings of the motor, produces a braking torque on the rotor which ultimately cannot be uncontrolled from outside the motor.
One remedy is to decouple the motor from the drive end in the event of a short circuit, so that the braking torque generated cannot be transmitted to the drive end and, in the case of a wheeled vehicle, can therefore not act on a wheel. In cited Utility Model Publication No. DE 20 2009 014 490 U1, this problem is solved specifically in that the permanent magnet synchronous motors used act on the drive shaft via freewheels which, by design, never transmit torques in an undesired direction, such as in the case of a braking torque. This has the disadvantage that in situations where a torque in such direction is required, this torque cannot be provided directly by the synchronous motors.